Electromechanical power transfer systems for aeronautical applications may integrate main and auxiliary engine start functions with onboard electric power generating equipment. A conventional brushless, WFSM is ideal for such an electromechanical power transfer system wherein it may serve as both a starter and a generator. It is a logical choice for modern variable frequency (VF) alternating current (AC) electric system architectures. A WFSM that serves as both a starter and a generator is representative of a class of variable speed motor drives in the start mode of operation that uses a solid-state power converter to process typically high potential direct current (DC) electric power into VF AC electric power suitable for driving the variable speed AC electric machine.
Aeronautical applications typically use a brushless WFSM, which is actually three electric machines using a common shaft in the same housing. The three electric machines typically include a permanent magnetic generator (PMG), an exciter and a main machine (MM). The common shaft functions as a rotor that turns inside various sets of stator windings. In some applications, the PMG is installed on another shaft that is geared to the main shaft containing the exciter and MM. The rotor may be turned by an engine or a gear box or a gear train. The PMG portion of the WFSM is formed from a section of the rotor having permanent magnets in it and a stator with a three phase winding, thereby, as the rotor turns, the PMG generates AC power because the rotating magnets induce AC power in the stator. The exciter consists of a rotor with AC windings and a stator with windings. The stator can have DC windings set up in a salient pole configuration or AC windings setup in a three phase configuration similar to an induction machine. During starter/motor mode, the exciter stator is excited with AC power that can be sourced from the generator control unit (GCU) or an aircraft bus. If the GCU is sourcing the AC power, GCU power is sourced from the batteries, ground power or an existing aircraft bus. During generate mode, the GCU rectifies the AC power from the PMG to DC power for use in the GCU control circuits and provides MM excitation via a DC to DC converter. The GCU will source the exciter with DC power in order to excite the MM. When the exciter is energized, in both starter/motor and generate modes, the stator windings form magnetic north-south pole pairs. Because the exciter rotor windings are three phase, the output power generated is continuous three phase AC in both modes of operation. A rotating rectifier is often included to convert the exciter AC output to DC for presentation to the MM. The MM has a rotor with a DC winding and a stator with an AC winding. Thus, as the rotor rotates, it generates an electromotive force (EMF) and produces power.
As described previously, the WFSM may be operated in generate mode or a starter/motor mode. Operation of the WFSM in the starter/motor mode constitutes a variable speed motor drive utilizing a solid-state power converter to process typically high potential DC electric power to provide variable frequency AC power input to the WFSM. For operation of a WFSM as a variable speed motor drive it is necessary to know the rotational position of the WSFM main rotor to control the solid-state power converter to meet motor performance requirements. Previous systems used position sensors (e.g., resolvers) to determine rotor position at low speed. During high speed operation, any stage of the three stage WFSM can be utilized to determine rotor position. Some systems have now replaced resolvers by self sensing rotor position using one or more components of the WFSM for low speed operation. In order to replace the position sensor, the component used as a replacement to a position sensor must have persistent excitation and must have saliency. Saliency can be defined as the difference between the inductance in the rotor quadrature axis (Q axis) and the inductance in the rotor direct axes (D axis), and these inductance values are generally controlled by how much magnetic field flows through a certain area. Thus, saliency is the variance between Q axis inductance and D axis inductance. This variance imprints spatial harmonics onto the waveforms, which are rotor position dependent, allowing for position determination.
One known technique of estimating the position of components in the WFSM involves superimposing a carrier voltage signal upon a fundamental control voltage signal. A controller generates the fundamental control signal, which modulates an AC power source that drives the WFSM to produce rotational torque. As the carrier voltage signal is a relatively high-frequency signal, the carrier voltage signal does not substantially affect the fundamental control signal driving the motor. The technique of estimating the angular position of the rotor is often referred to as the carrier injection sensorless (“CIS”) method and is described in U.S. Pat. No. 5,585,709, and the entire disclosure of this patent is incorporated herein by reference in its entirety.
The CIS method has proven useful but it has shortcomings. For example, the CIS method may undesirably place an increased current carrying burden on some components. Thus, other techniques have been developed, such as a technique that measures and utilizes current harmonics of a PMG rather than the current harmonics induced by a carrier voltage signal. An example of this technique is described in U.S. Pat. No. 8,362,728, and the entire disclosure of this patent is incorporated herein by reference in its entirety. In the example of the CIS technique, the PMG is excited with only the CIS signal because it is not utilized in starter mode, however the position sensing technique is consistent with CIS methods and is dependent on the PMG saliency.
In previous designs the PMG has consisted of a single phase flux switching generator (FSG) or a three phase PMG. For both designs, the PMG output is AC. The single phase FSG provides a simple construction. However, the obvious disadvantage of a single phase FSG is that it is single phase, so it does not produce continuous power when converted from AC to DC. As a result, the GCU must include a very large filter in comparison to a GCU sourced with three phase power. Additionally, the load regulation is very poor in the single phase system.
The three phase PMG typically consists of a stator with three phases and is a surface mounted permanent magnet rotor. The magnets are mounted onto a hub with a containment band installed around the outer diameter for magnet retention. Although three phase PMG's provide continuous 3 phase power to the GCU and reduce the size of the machine, its rotor magnets tend to be brittle and require careful handling during all piece part and next higher assembly phases. During assembly, the magnets can easily chip or crack leading to weak points and creating contamination in the machine. The operating speeds require a containment band to prevent the magnets from flying off the rotor. The containment band is non-magnetic (e.g., titanium, inconel) to reduce leakage inductance and losses. The bands are expensive and difficult to produce due to the hardness of the non-magnetic materials. Overall cost can be high due to the magnet material and the containment band material.
Also, three phase PMG's tend to be ineffective as sensors because the D and Q axes inductances are almost identical in surface mounted permanent magnet rotors. Without a clear difference and/or isolation of the D axis from the Q axis, it is difficult to precisely identify the rotor position. Self-sensing schemes for AC machines required saliency in order to determine rotor position.